Hodgkin's Law of Parallel Planetary Development was a biological theory first postulated by A.E. Hodgkin. The theory was that similar planets with similar environments and similar populations tended to gravitate toward similar biological developments over time. Although initially applicable only to biology it was later expanded to include a tendency to move toward similar sociological developments as well with sentient beings. (ENT: "Strange New World"; TOS: "Bread and Circuses")

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Just as the finches of the Galapagos Islands provided the crucial biological clues that Charles Darwin used to develop the theory of evolution, the termites of Loracus Prime were the inspiration behind Hodgkin's law of parallel planetary development. The theory was the brainchild of biologist A.E. Hodgkin, who first visited the planet Loracus Prime as part of a science survey mission. As Hodgkin noted in his log, the native termite life of Loracus was remarkably similar to that of Earth, Vulcan, and several other Class M worlds. At first, Hodgkin considered the possibility that termites were brought to Loracus by early space travelers, or even that they were a meteorite-borne species, but he quickly realized that Loracus Prime's location in the middle of the Gagarin Radiation Belt ruled out either possibility. In fact, travel to the Loracus system had been all but impossible until the then-recent era of relatively clear stability of Loracus's star.

After careful testing of the termites' DNA, Hodgkin determined that the genus was clearly native to Loracus. This set up the question: why there was such an amazing similarity to terrestrial termites? Over the next decade and a half, Hodgkin slowly puzzled out the theory that would rock the biological world as profoundly as Darwin's had centuries earlier. (ENT: "Strange New World")

The next development in the theory was the realization that there was a tendency toward sociological as well as biological similarities where environmental conditions were similar. For instance, by the 2260sStarfleet and other exploratory organizations had discovered numerous planets with humanoid populations that shared certain social constructs. Family units, spoken languages, space travel, dispute resolution through an organized legal process, organized war waged by governments – all of these were aspects of society that appeared to transcend any one single planet's societal development. (citation needed • edit)

The Hodgkin theory was adapted to explain the observed instances of similarities in societies that had never had previous contact with each other developing along similar lines. The theory did not require identical development of a society, it simply offered an explanation for similarities. Finding nearly identical development was "virtually impossible" (as Spock remarked about the planet Ekos), and when found to be close to identical was viewed as an "amazing" example of Hodgkin's Law (as CaptainKirk remarked about the society on the planet 892-IV). (citation needed • edit)

Beginning with the lowly termites on Loracus Prime, there have been numerous examples of parallel biological development observed over time. Animals as diverse as the dog and the targ, the Capellan power-cat and the sehlat all independently developed fur, mouths, four legs, heads, tails, eyes, and other similar attributes, despite none of them being native to the same planet.

Most striking is the apparent biological preference for the humanoid form for sentientlifeforms, observed across species as diverse as Vulcans, Klingons, Denobulans, Humans, and Ocampa. Part of this is doubtless due to the actions of an ancient humanoid species, one of the oldest known sentient species, and possibly the first humanoid race in the Milky Way Galaxy. According to a message the humanoids left behind, encoded in the DNA of various humanoid species and discovered in 2369, they had seeded many worlds, including Earth, with DNA "seed codes" that encouraged the development of similar humanoid life. (TNG: "The Chase")

Many worlds have independently developed in ways that support the application of Hodgkin's law to societies. Several worlds in particular visited by the USS Enterprise in the 2260s showed exceptional examples of societies that developed in ways that were extreme examples of Hodgkin's Law.

Prior to the 23rd century, Omega IV had developed two cultures similar to those found on Earth: Americans and Asian Communists. The state of the cultures reached levels similar to those of Earth's 20th or 21st century before devastating bacteriological wars brought about a collapse of both civilizations. While independently developing an economic system based on capitalism and profit is not itself remarkable (e.g., the Ferengi), what set this planet apart for purposes of the Hodgkin theory was the extraordinary similarity of names, symbols, and even documents to those found on Earth. The name of one culture – the Yang – was clearly a derivation of "Yankee", while the name of the other culture – the Kohm – was equally clearly a derivation of "Communist". The Yangs had, prior to the wars, developed a flag and even a preamble to a Constitution that were virtually identical to those of the United States of America in the early 21st century on Earth.

The planet 892-IV had a land-mass/ocean ratio similar to Earth, although it possessed a significantly different topography. But in an "amazing" example of Hodgkin's law, it saw the development of a society that was remarkably like that of Earth's Roman Empire. It also independently developed its own Jesus Christ and associated religion based on his being the son of God. A major difference, however, was that unlike Earth the Empire had not fallen by the time of the Enterprise visit in 2268. At that time the Roman Empire dominated the planet, but at a 20th century level of technology. (TOS: "Bread and Circuses")

Captain R.M. Merik asserted that 892-IV had not experienced war since their equivalent of the 16th century, implying the period this Roman Empire established domination of 892-IV. The planet also speaks English rather than Latin (Spock comments several times on the use of "colloquial 20th-century English"), but the universal translator may have played a role. Vernacular languages were used throughout the Roman Empire, making an English speaking province believable.

Unique in the annals of parallel planet development, in 2267 the USS Enterprise discovered Miri's homeworld. It was a world physically identical to Earth: it had the same mass, circumference, density, atmosphere, flora, and fauna. Even the topography was identical. The inhabitants of this world were virtually indistinguishable from Humans as well, although a biological accident had killed every single adult and left the remaining prepubescent children with extraordinarily long lifespans. The planet's inhabitants had reached at least a level of technology and architecture typical of the United States circa 1960 prior to the disaster. (TOS: "Miri")

Cloud cover on Miri's homeworld appeared to be much less dense than on Earth, but that was most likely a means of showing similarity to an audience than an actual portrayal of the planet as envisioned. A canon explanation as to whether this planet and Earth simply developed naturally, or whether external forces were at work was never provided. The William Shatner novel Preserver featured a return to this planet, wherein it was revealed the planet had been duplicated from Earth on a subatomic level by the Preservers.

Hodgkin was originally concerned that the similarities he saw regarding the termites on Loracus Prime were the result of contamination. While he ultimately dismissed that as a possibility, later discoveries have shown that there was far more biological and cultural contamination in the distant past than was known during his time.

Some major events that might provide either alternative or supplementary explanations for developments thought to be examples of Hodgkin's law include numerous visitors to Earth and other planets in the distant past (including Apollo, the Megans, and Kukulkan), as well as:

An ancient humanoid species, one of the oldest known sentient species, and possibly the first humanoid race, seeded the oceans of many Milky Way Galaxy worlds, including Earth's, with "DNA codes" that encourage the development of similar humanoid life. A message the humanoids left behind describing this was discovered in 2369. (TNG: "The Chase")

In the Permian-period, Eryops, the last common ancestor of cold-blooded and warm-blooded organisms, lived on Earth. A cold-blooded, sentient, descendant species of Eryops called the Voth was discovered in the Delta Quadrant in 2373. (VOY: "Distant Origin")

Although much was made in the episode of a possible Voth civilization on Earth, the episode never concluded that it definitely existed. It is equally plausible that the Preservers relocated the young species as either Voth or Eryops to the Delta Quadrant. And if the Voth arose on Earth and migrated out from there, they may have contaminated a number of planets with terrestrial DNA.

Sargon's species colonizes many worlds across the galaxy, possibly including Vulcan as well as potentially a number of other planets bearing Human-like or Vulcan-like life in the present day. (TOS: "Return to Tomorrow")

In an attempt to prevent mankind from destroying itself in the distant future, an alien species took Humans to be raised on a planet located 1,000 light years from Earth. A descendant of these Humans, Gary Seven was discovered by the crew of the Enterprise manipulating events on Earth in 1969. (TOS: "Assignment: Earth")

"Hodgkin's Law" was a writer's device used to reconcile TOS science-fiction themes with obvious budget limitations. Gene Roddenberry wrote in his original pitch:

The "Parallel Worlds" concept makes production practical by permitting action-adventure science fiction at a practical budget figure via the use of available "earth" casting, sets, locations costuming and so on.

As important (and perhaps even more so in many ways) the "Parallel Worlds" concept tends to keep even the most imaginative stories within the general audience's frame of reference through such recognizable and identifiable casting, sets and costuming."

Later incarnations of Star Trek allowed for a wider variety of alien physiology, but still, budget limitations meant a dependence on basically humanoid forms, cunningly crafted foreheads notwithstanding. The Ancient Humanoids, introduced in TNG: "The Chase", served a similar purpose as "Hodgkin's Law", giving a source for possibly all of the Galaxy's remarkably Human-shaped life as well as the ability for alien species to cross-breed.

The explanation for the similarities between Humans and many alien species bears a number of similarities to non-canonical explanations of the mirror universe. In several novels and comic books, writers have attempted to define a point at which the mirror universe and our universe diverged, given the overwhelming similarities in physical appearance of people and places.